The VOR system as such is employed extensively throughout the world and it is operated to provide an aircraft with flight path bearing information. Two signals are radiated by a VOR antenna to produce a rotating field in space, one signal being referred to as a reference phase signal which is radiated omnidirectionally and the other signal being referred to as a variable phase signal which has a phase which varies linearly with azimuth angle. Bearing information is derived by detecting the phase difference between the reference and variable phase signals as received by an aircraft flying toward or from the VOR site.
The reference phase signal is generated as a radio frequency (r.f.) carrier which has a frequency falling within the region 108-118 MHz and which is amplitude modulated by a 30 Hz frequency modulated 9960 Hz subcarrier. The variable phase signal comprises a portion of the r.f. carrier from which the modulation is eliminated and, when radiated, is space amplitude modulated at 30 Hz. The space modulation is achieved by feeding the radiating antenna so as to produce a field which rotates at 1800 RPM or, expressed otherwise, at 30 Hz.
The field which is obtained by combining the reference and variable phase signals is a rotating field containing reference phase and variable phase information.
Bearing information is derived and indicated by a receiver within an aircraft. After processing in the r.f. stage of the receiver and subsequent detection, the received reference and variable phase signals are processed in separate channels and are applied as separate inputs to a phase comparator. Bearing information relative to the VOR site is indicated by the phase difference between the reference and variable phase signals.
The above described system, termed Conventional VOR (CVOR), is susceptible to multipath errors due to reflections from surface irregularities above or below the ground plane of the VOR site. Such surface reflections may be due to the existence of varying topography or to the interfering presence of buildings, fences, trees, etc., and multipath errors flow from vector addition of direct and reflected field components of the field into which the aircraft flies.
Certain multipath errors are minimised by use of a Doppler VOR (DVOR) system in which a 30 Hz amplitude modulated carrier signal (reference phase signal) is radiated omni-directionally and an azimuth dependent signal (variable phase signal) is generated in space (using the Doppler principle) by radiating 9960 Hz frequency modulated side band(s) of the carrier from an array of sequentially switched antennas which are located concentrically about the reference phase signal antenna. A typical DVOR array has 50 antennas disposed around a horizontal aperture of about 5 wavelengths (13.5 meters) diameter.
The DVOR system very successfully reduces multipath in many cases, but it suffers from the following disadvantages:
(a) It does not protect against lobing of the vertical field pattern, PA1 (b) It provides no real improvement over the CVOR system PA1 (c) It involves a high capital and installation cost.
for reflections from objects within .+-.10.degree. of line of sight, and
The present invention is applicable to a CVOR system and it is directed to the minimization of multipath errors which result from reflections due to radiation into negative angles.
Many en-route VOR sites are placed on small hilltops in order to ensure adequate range in the presence of surrounding hills. However, a small ground plane only is then available and the VOR signal radiates strongly into the region below the horizon. Measurements on difficult (elevated, short ground plane) sites in Australia have shown that the negative angle field can be at least as strong as the field at positive angles, and it is this negative angle radiation that causes most of the multipath errors on such sites. Early work on VOR systems recognised this problem very clearly, but in more recent times the existence of negative angle radiation has largely been ignored or, at least, disregarded by designers (and users) of VOR systems.
Typical problem sites have a ground plane radius (in at least one direction) in the order of 4.lambda. to 8.lambda. (10.5 to 21 m.) and significant multipath errors can occur with sites having a ground plane radius up to 20.lambda.. However where a large counterpoise (of radius greater than, say, 20.lambda.) is available multipath errors resulting from any negative angle field radiation are normally acceptable, so the present invention (as hereinafter defined) may be considered as having greatest application to elevated sites which have a ground plane radius less than about 50 m.
Attempts are known to have been made to obviate multipath errors which result from the abovementioned problem sites, by use of vertically stacked antennas. Thus, two-stack and five-stack antenna systems have been developed in an attempt to produce a field pattern which provides a null near zero degrees of elevation. The idea behind such systems presumably is to create a free space field pattern which (theoretically) cuts-off at zero degrees in the far field and which will reduce illumination of the surrounding (negative angle) terrain. However it is thought that the (far field) free-space design approach cannot be entirely satisfactory because it ignores any field contribution from partially formed images that will occur in the short ground plane that is always available. The partially formed images exist as a result of ground currents being induced in the ground plane.